This invention relates to electroluminescent phosphors and more particularly to such phosphors having increased color-stability during subsequent firing operations.
Electroluminescent phosphors are materials that emit light in the presence of an alternating current electric field. Such phosphors are known. A particular example is a ZnS:CuCl material available as Type 723 from Osram Sylvania Products Inc. in Towanda, Pa.
When placed in the presence of a suitable electric field, lamps employing these phosphors generally emit light m the blue-green area of the visile spectrum, i.e., having color coordinates (on a standard C.I.E. diagram) of x=0.184 and y=0.435. When used in some lamps it has occasionally been found necessary to provide these phosphors with coating to prevent moisture degradation. These coatings are applied by treating the phosphors, usually by a vapor deposition process, at temperatures above 175xc2x0 C. This subsequent processing has been known to cause an unwanted and detrimental shift in the color emission of the phosphor, causing it to emit more in the yellow, that is, to a material having coordinates in the range of x=0.193 to 0.199 and y=0.471 to 0.444, the shift being dependent upon the temperature range of the firing.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art and to provide a phosphor having increased color-stability.
These objects are accomplished, in one aspect of the invention, by a method of increasing the color-stability of an electroluminescent phosphor which comprises the steps of forming the phosphor and then firing the formed phosphor in a vacuum in the presence of a material selected from the group of Eu, Ce, and Sm for a time sufficient to provide said increase in color-stability.